1. Field of the Invention
The invention relates to the field of transceivers such as those used to transmit and receive data over a power line and particularly where the transceiver includes two channels operating at different frequencies.
2. Prior Art
Transceivers for transmitting and receiving data over power lines as well as over other media, are well-known. Generally, binary data is transmitted by modulating a carrier frequency. Echelon Corporation of Palo Alto, California manufactures and sells its PLT-21 transceiver which transmits and receives data with a carrier frequency of approximately 131 kHz over power lines. Different aspects of this transceiver are described in U.S. Pat. Nos. 5,471,209; 5,463,662; 5,553,081; 5,703,766; 5,701,240 and USSN: 08/661,136; filed Jun. 10, 1996 which is assigned to the assignee of the present invention.
The present invention grew out of an improvement to the PLT-21 transceivers, specifically an improvement to add a second channel. The second channel, as will be discussed, is used to transmit data when it appears that the first channel is blocked, for instance, by noise on the power line.
In U.S. Pat. No. 5,355,114 a transceiver is described that broadcasts data simultaneously on two channels and then selects between the channels by examining the noise on the channels. As will be seen this is different than the present invention where data is only transmitted in a single channel at any given time.
A method for transmitting data from a transmitter having a first and second channel is described. The data is transmitted at a first power level when the data is transmitted on the first channel without transmitting any signal at all on the second channel. When it appears that the first channel is blocked, a retry is made on the second channel with data being transmitted on the second channel at a power level slightly less than the power level used on the first channel. A pilot or synchronization signal is simultaneously transmitted on the first channel at a somewhat lower power level. This pilot tone keeps the network timing of transceivers having only the first channel, synchronized and thereby prevents them from transmitting data onto the line at the same time that second channel signals are on the line. If transmissions were to occur on the first channel they would interfere with transmissions being made over the second channel.
Other aspects of the present invention will become apparent from the following Detailed Description.